<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>ajax链式调用</title>
</head>
<body>
    
</body>
<script>
    function ajax(url) {
        return new Promise((resolve, reject) => {
            let xhr = new  XMLHttpRequest()
            xhr.open('get', url)   //链接
            xhr.send()             //发送资源
            xhr.onreadystatechange = function () {  //进行状态监听
                console.log(xhr);
                if (xhr.readyState === 4) {
                    if (xhr.status >= 200 && xhr.status < 300) {
                        let res = JSON.parse(xhr.responseText)  //json 字符串
                        resolve(res)
                    } else {
                        reject(xhr.responseText)
                    }
                }
            }
        })
    }

    ajax('./01.json')
    .then((res) => {
        console.log(res);
        return ajax('./02.json')
    })
    .then((res) => {
        console.log(res);
    })
    .catch((err) => {
        console.log(err);
    })
</script>
</html>